Polyurea coatings for objects of metal glass wood or plastic

ABSTRACT

The polyurea resulting from curing polyfunctional isocyanates having at least three groups with a N,N&#39;-dialkylmethylenedianiline are broadly useful as coatings on the surface of objects made of metal, wood, glass, or plastic. Such coatings may be made based on either a 1-pack system or 2-pack system. The diamine N,N&#39;-di(2-butyl)4,4&#39;-methylenedianiline is a particularly useful diamine. Biurets are an especially desirable class of polyisocyanates for the polyureas of this invention.

BACKGROUND OF THE INVENTION

As a subclass of commercially available polymers, polyurethaneelastomers have several properties whose advantages confer uniquebenefits on these products. Typically, polyurethanes show high abrasionresistance with high load bearing, excellent cut and tear resistance,high hardness, resistance to ozone degradation, yet are pourable andcastable. Compared to metals, polyurethanes are lighter in weight, lessnoisy in use, show better wear and excellent corrosion resistance whilebeing capable of cheap fabrication. Compared to other plastics,polyurethanes are non-brittle, much more resistant to abrasion, andexhibit good elastomeric memory. Polyurethanes find use in such diverseproducts as aircraft hitches, bushings, cams, gaskets, gravure rolls,star wheels, washers, scraper blades, impellers, gears, and drivewheels.

Part of the utility of polyurethanes derives from their enormousdiversity of properties resulting from a relatively limited number ofreactants. Typically, polyurethanes are prepared on site by curingurethane prepolymers, which are adducts of polyisocyanates andpolyhydric compounds. A large class of such prepolymers areapproximately 2:1 adducts of a diisocyanate, OCN--Y--NCO, and a diol.Another class of prepolymers, especially pertinent to this application,result from the reaction of diisocyanates with a limited amount of waterto give as prepolymers principally biurets with smaller amounts ofhigher condensation products according to the reaction, ##STR1##Although Y is susceptible of great variety, usually being a divalentalkyl, cyclohexyl, or aromatic radical, in fact the most availableurethane prepolymers are made from toluene-2,4-diisocyanate (TDI) ormethylene-4,4'-diphenylisocyanate (MDI), although the biurets show asomewhat greater structural variation.

The polyurethane elastomers and polyureas are formed by curing theurethane prepolymer or biuret, respectively. Curing is the reaction ofthe terminal isocyanate groups of the prepolymer or biuret with activehydrogens of a polyfunctional compound so as to form high polymersthrough chain extension and, in some cases, crosslinking. Diols,especially alkylene diols, are the most common curing agents, especiallyfor MDI-based urethane prepolymers, and representing such diols with thestructure HO--X--OH, where X is an organic moiety, most usually analkylene group, the resulting polymer has as its repeating unit,

    (--Y--NHCO.sub.2 XO.sub.2 CNH--Y--NHCO.sub.2 --X--O--CONH--)

Where a triol or a higher polyhydric alcohol is used crosslinking occursto afford a nonlinear polymer. Where biurets are the prepolymercrosslinking occurs even with diols, since the biuret itself is at leasttrifunctional.

Although other polyfunctional chemicals, especially diamines, aretheoretically suitable as a curing agent, with but a few exceptions nonehave achieved commercial importance. The major exception is4,4'-methylene-di-ortho-chloroaniline, usually referred to as MOCA, acuring agent which is both a chain extender and a crosslinker. TDI-basedurethane prepolymers typically are cured with MOCA, and the resultingproducts account for perhaps most of the polyurethane elastomer market.One reason that polyhydric alcohols generally have gained acceptance ascuring agents is that their reaction with urethane prepolymers issufficiently fast to be convenient, but not so fast as to make itdifficult to work with the resulting polymer. In casting polymers it isdesirable that the set-up time be reasonably short, yet long enough forthe material is be cast into molds. This property is conventionallyreferrec to as pot life. Generally speaking, diamines react withprepolymers, and especially MDI-based prepolymers, so quickly that theyare not usable as curing agents. However, primary aromatic diamines withelectronegative groups on the aromatic ring, or with alkyl groups orthoto the amino moiety, exhibit sufficiently decreased reactivities withsome prepolymers as to afford a desirable pot life, hence the use of,for example, MOCA as a curing agent for TDI-based urethane prepolymers.However, MOCA and other of the aforementioned diamines still remain tooreactive to be used, for example, with MDI-based urethane prepolymers.

Previously only primary aromatic diamines seem to have been used ascuring agents. Presumably this is because secondary diamines wereexpected to have an unacceptably long pot life, and because they couldact only as chain extenders with urethane prepolymers in contrast to thecrosslinking capabilities of primary diamines. Recently, however, wehave found that certain N,N'-dialkyl-4,4'-methylenedianilines aregenerally effective curing agents for a broad range of urethaneprepolymers. The resulting polyurethanes often have the advantage ofbeing thermoplastic rather than thermosetting, thereby making themespecially useful as coatings, adhesives, and sealants. We also havefound that such aromatic alkyl diamines are effective curing agents forbiurets which, because of the latter's trifunctionality (and higherfunctionality), are crosslinked by these diamines to afford polyurearesins with quite desirable properties.

Polyurethanes find extensive application as coatings and adhesives.Among the properties of polyurethanes particularly desirable in thecoating art are their chemical resistance, light stability, flexibility,toughness, weatherability, moisture resistance, abrasion resistance,gloss and color retention, and impact resistance. We have found thatbiuret prepolymers cured with the secondary amines we previouslydescribed afford resins which are particularly suitable as coatings, andthis application is directed toward that use.

SUMMARY OF THE INVENTION

The object of this invention is to use particular polyureas as coatingsand adhesives. An embodiment comprises the use of polyureas resultingfrom curing a diisocyanate-terminated biuret prepolymer with an aromaticalkyl diamine. In a more specific embodiment the alkyl is a secondaryalkyl moiety. In a still more specific embodiment the biuret is thatfrom 1,6-hexamethylene diisocyanate. Other embodiments will be apparentfrom the description which follows.

DESCRIPTION OF THE INVENTION

The invention herein is the use as coatings of polyureas resulting fromcuring a polyisocyanate which is at least trifunctional, especially adiisocyanate-terminated biuret prepolymer, with an aromatic alkyldiamine, especially those with a secondary alkyl moiety. It has beenfound that such polyureas have properties which lend themselvesespecially well do the coating art, hence these polyureas findparticular application as coatings.

The isocyanates used in the practice of this invention arepolyisocyanates which are at least triisocyanates. Prime examples ofsuch polyisocyanates are the adducts of water and a diisocyanate in sucha ratio as to give a biuret as the predominant product according to theequations written above. Thus, the biuret has the formula, ##STR2## Thestructural unit Y is capable of great diversity, and in one commerciallysignificant variant Y is a polymethylene moiety, --(CH₂)_(n) --, where nis an integer from 2 to about 10, with n=6 being especially important.In another variation Y is a divalent aromatic moiety, its completelysaturated cyclic (cyclohexyl) counterpart, or a divalent aralkyl moiety.Examples of the resulting diisocyanates, OCN--Y--NCO, which may be usedin the practice of this invention include phenylene diisocyanate,toluene diisocyanate, xylene diisocyanate, 1,5-naphthalene diisocyanate,chlorophenylene 2,4-diisocyanate, bitoluene diisocyanate, dianisidinediisocyanate, tolidine diisocyanate and alkylated benzene diisocyanatesgenerally; methylene-interrupted aromatic diisocyanates such asmethylenediphenyl diisocyanate, especially the 4,4'-isomer includingalkylated analogues such as 3,3'-dimethyl-4,4'-diphenylmethanediisocyanate; such hydrogenated materials as cyclohexylene diisocyanate,4,4'-methylenedicyclohexyl diisocyanate; mixed aralkyl diisocyanatessuch as the tetramethylxylyl diisocyanates, OCN--C(CH₃)₂ --C₆ H₄ C(CH₃)₂NCO, and the diisocyanate popularly referred to as isophoronediisocyanate, which is 3,3,5-trimethyl-5-isocyanatomethylcyclohexylisocyanate; and polymethylene isocyanates as 1,4-tetramethylenediisocyanate, 1,5-pentamethylene diisocyanate, 1,6-hexamethylenediisocyanate, 1,7-heptamethylene diisocyanate, 2,2,4- and2,4,4-trimethylhexamethylene diisocyanate, 1,10-decamethylenediisocyanate. The biuret prepolymers are adducts of 1 mole water toabout 3 molar proportions of diisocyanate, with the proportions commonlyranging between 1:1 and about 1:6.

The polyurea elastomer is made by reacting the polyisocyanate with asecondary aromatic alkyl diamine of the structure. ##STR3## Each alkylgroup, R, contains from 4 up to about 20 carbon atoms. Alkyl groupscontaining from 4 to 10 carbon atoms, and especially from 4 to about 8carbon atoms, are particularly preferred. The alkyl group may be aprimary, secondary, or tertiary alkyl group, although when the alkyl istertiary there is the risk that cure time may be too long to becommercially acceptable. Secondary alkyl groups are preferred, and amongthese the secondary butyl group is particularly preferred. Of thepositional isomers possible the 4,4'-methylenedianilines are mostdesirable.

Defining an equivalent of diamine as an amount which furnishes as manyamino groups as there are isocyanate groups in the polyisocyanate orbiuret prepolymer, from about 0.80 to about 1.2 equivalents of diamineare used in curing, with the range from about 0.85 to about 1.1 beingthe more usual one. Since each amino group has only one hydrogen, suchsecondary amines act by themselves only as a chain extender and not as acrosslinker. However, since the polyisocyanates are at leasttrifunctional the resulting polyurea will be extensively crosslinked.The curing mix may contain other materials, including polyols, inaddition to, or partly replacing, the diamines of this invention. Wherea polyol is present it typically will replace from about 5% to about 35%of the diamine. It is to be understood that such a curing mixture iscontemplated as being within the scope of this invention.

The initial reaction between the polyisocyanate and the diamine isbetween about 75 deg and about 120 deg C. The temperature is chosen, inpart, to afford a convenient pot life, that is, the time interval frommixing the diamine and polyisocyanate until the mixture is verydifficult to pour. The elastomer is then cured to a tack-free state byheating at the same temperature range for an additional period fromabout 2 to about 24 hours.

The polyureas of this invention may be used as a coating for objects ofmetal, glass, wood, or plastic, for one feature of these polyureas istheir ability to adhere strongly to a great variety of surfaces. Suchversatility makes the materials of this invention particularly valuable.

The polyurea surface coatings described here may be applied as 1-pack or2-pack systems. See R. Heath, Urethanes Technology, March, 1985, 17-20.In the 1-pack system the fully reacted polyurea is either in solution oris dispersed in a suitable medium. Examples of solvents which aresuitable for use in the practice of this invention are aromaticsgenerally, such as benzene, toluene, the xylenes, ethylbenzene,propylbenzene, and so forth; ketones, especially acetone andmethylethylketone; and halogenated solvents such as chloroform, carbontetrachloride, trichloroethylene, and so forth. The nature of thesolvent is not particularly critical so long as it is unreactive withthe polyurea, although a solvent which can be readily evaporated isdesirable. The same attributes apply to the dispersing medium, where asuspension of the polyurea is used instead of a solution, with wateroften being a suitable dispersing medium.

The solution or dispersion of the polyurea may be applied to the surfaceof the object in any convenient way. Often such material is sprayed on asurface, although it may be painted on, the surface may be dip-coated,roller coated, and so on. Such methods of application are well known inthe art and need not be elaborated upon further. After the coating hasbeen applied the solvent or dispersing medium is evaporated, generallyat a somewhat elevated temperature depending upon the nature of thesolvent or dispersing medium. Generally, such temperatures do not exceedabout 120 deg C.

In the 2-pack system a polyisocyanate and a suitable diamine are appliedto a surface of the object, and the mixture is reacted or cured at anelevated temperature to form the polyurea. Conventionally, thepolyisocyanate and the curing agent diamine are mixed immediately beforeapplying to the surface of the object. However, it is possible to applythe polyisocyanate and the diamine separately. In either event, thefinal polyurea results from their reaction at elevated temperature,i.e., post-application curing is necessary. Curing occurs between about75 deg and about 130 deg C., although higher temperatures may beemployed if very short cure times are desired.

The example which follows is only illustrative of the invention, whichis not to be limited thereby in any way.

Preparation of Polyurea Coatings.N,N'-di(2-butyl)-4,4'-methylenedianiline was mixed with acetic acid(0.05% based on total weight) and then mixed with Desmodur N-100 atdifferent NCO indexes. Desmodur N-100 is an adduct of hexamethylenediisocyanate and water of equivalent weight 190, supplied by MobayChemical Co., and which is often used as an industry standard. Dryxylene was added to obtain a uniform solution with about 50% solidscontent. This transparent solution was coated on the clean aluminumpanels using a doctor blade and baked at 130 deg C. for 30 minutes. Astandard polyurea coating made from Desmodur N-100 and Desmophen650A-65, a saturated polyester resin of equivalent weight 325 suppliedby Mobay Chemical Co., was used as a control.

The sward and pencil hardness of the polyurea coatings were measured ona Sward Rocker and Brumbaugh pencil. The solvent resistance wasdetermined on the surface of the polyurea coatings by using the doublerub technique with MEK or xylene. The number of double rubs was recordedto express the solvent resistance properties. The impact resistance ofthe polyurea coatings was measured on a Gardner-SPI Modified VariableHeight Impact Tester using both the direct and indirect techniques. Thechemical resistance was measured by placing the panels with polyureacoating and wax (to protect the metal square) into a 10% NaOH or HClsolution at room temperature for one week. Any change of the surface(such as transparency, color and gloss, etc.) was recorded.

Table 1 summaries some properties of coatings prepared as previouslydescribed. The adduct (190 parts by weight) was mixed with differentamounts (in parts by weight) of the secondary diamine to afford coatingsof varying NCO/NH ratio. As shown there, the coating fromN,N'-di(2-butyl)-4,4'-methylenedianiline with different NCO indexesexhibits better Gardner impact resistance and hardness than the control.The chemical resistance and the solvent resistance are the same as thecontrol, except for the MEK solvent resistance properties.

                  TABLE 1                                                         ______________________________________                                        Properties of Polyurea Coatings Based on Desmodur N-100                       With N,N'--di(2-butyl)-4'-methylenedianiline or Desmophen                     650A-65                                                                       ______________________________________                                        Formulation  1        2        3      4                                       Desmodur N-100                                                                             190      190      190    190                                     Unilink 450  175.8    158.2    143.8  --                                      Desmophen    --       --       --     325                                     650A-65.sup.(1)                                                               Dibutyltin dilaurate                                                                       --       --       --     0.26                                    (0.05%).sup.(2)                                                               Acetic acid (0.05%).sup.(2)                                                                0.18     0.17     0.17   --                                      Xylene (50%).sup.(2)                                                                       183      174      167    --                                      Ethyl acetate (50%)                                                                        --       --       --     288                                     NCO/NH       0.9      1.0      1.1    --                                      NCO/OH       --       --       --     1.0                                     Baking condition,                                                                          30/130   30/130   30/130 30/130                                  min./°C.                                                               Hardness                                                                      Sward        30       26       16     10                                      Pencil       HB       HB       HB     HB-B                                    Gardner Impact                                                                Direct, lb. in.                                                                            140      140      140    140                                     Indirect, lb. in.                                                                          140      120      140    80                                      Solvent resistance.sup.(3)                                                    MEK          Pass/70  Pass/70  Pass/50                                                                              Pass/100                                Xylene       Pass/100 Pass/100 Pass/100                                                                             Pass/100                                Chemical resistance.sup.(4)                                                   10% HCl × RT ×                                                                 Pass     Pass     Pass   Pass                                    One Week                                                                      10% NaOH × RT ×                                                                Pass     Pass     Pass   Pass                                    One Week                                                                      Thickness, mils                                                                            6        5        10     10                                      ______________________________________                                         .sup.(1) A viscous, saturated polyester resin supplied by Mobay Chemical      Co. as a 65% solution in ethyl glycol acetate, eq. wt. = 325, solids % =      65.                                                                           .sup.(2) Based on solids content.                                             .sup.(3) Using double rub with solvent, and the number of double rubs         without damage to the surface were recorded.                                  .sup.(4) The surface did not change after immersing in 10% HCl or NaOH fo     one week at room temperature.                                            

What is claimed is:
 1. A method of coating an object of metal, glass, wood, or plastic comprising applying to a surface of the object a solution or dispersion of a polyurea resulting from the reaction of a polyisocyanate having at least three isocyanate moieties with from about 0.8 to about 1.2 equivalents of a secondary aromatic diamine of the structure, ##STR4## where each alkyl group, R₁ and R₂, contains from 4 to about 20 carbon atoms, and evaporating the solvent or dispersing medium.
 2. The method of claim 1 wherein the polyisocyanate is a biuret of the structure, ##STR5## where Y is: a. a polymethylene chain, --(CH₂)_(n) --, with n being an integer from 2 to about 10; orb. a divalent aromatic radical; or c. a divalent cyclohexyl radical; or d. a divalent aralkyl radical.
 3. The method of claim 2 where Y is a hexamethylene moiety.
 4. The method of claim 2 where the diisocyanate precursor of the biuret is an aromatic diisocyanate selected from the group consisting of phenylene, toluene, xylene, naphthalene, methylenediphenyl, and 3,3'-dimethyldiphenylmethane diisocyanate.
 5. The method of claim 2 where the diisocyanate precursor of the biuret is a cyclohexyl diisocyanate selected from the group consisting of cyclohexyl, methylcyclohexyl, and methylenedicyclohexyl diisocyanate.
 6. The method of claim 2 where the diisocyanate precursor of the biuret is a diisocyanate which is tetramethylxylyl and isophorone diisocyanate.
 7. The method of claim 1 where the alkyl group contains from 4 to about 10 carbon atoms.
 8. The method of claim 7 where the alkyl group contains from 4 to 8 carbon atoms.
 9. The method of claim 1 where the alkyl is a secondary alkyl moiety.
 10. The method of claim 1 where the alkyl group is the 2-butyl group.
 11. The method of claim 1 where the diamine is a N,N'-dialkyl-4,4'-methylenedianiline.
 12. The method of claim 11 where the diamine is N,N'-di(2-butyl)-4,4'-methylenedianiline.
 13. The method of claim 1 where from about 5% to about 35% of the diamine is replaced by a polyol.
 14. A method of coating an object of metal, glass, wood, or plastic comprising applying to a surface of the object a polyisocyanate having at least three isocyanate moieties and from about 0.8 to about 1.2 equivalents of a secondary aromatic diamine of the structure, ##STR6## where each alkyl group, R₁ and R₂, contains from 4 to about 20 carbon atoms, and reacting the prepolymer with the diamine at a temperature from about 75° to about 130° C. so as to form a polyurea on said surface.
 15. The method of claim 14 where the polyisocyanate is a biuret of the structure, ##STR7## where Y is: a. a polymethylene chain, --(CH₂)_(n) --, with n being an integer from 2 to about 10; orb. a divalent aromatic radical; or c. a divalent cyclohexyl radical; or d. a divalent aralkyl radical.
 16. The method of claim 15 where Y is a hexamethylene moiety.
 17. The method of claim 15 where the diisocyanate precursor of the biuret is an aromatic diisocyanate selected from the group consisting of phenylene, toluene, xylene, naphthalene, methylenediphenyl, and 3,3'-dimethyldiphenylmethane diisocyanate.
 18. The method of claim 15 where the diisocyanate precursor of the biuret is a cyclohexyl diisocyanate selected from the group consisting of cyclohexyl, methylcyclohexyl, and methylenedicyclohexyl diisocyanate.
 19. The method of claim 15 where the diisocyanate precursor of the biuret is a diisocyanate which is tetramethylxylyl or isophorone diisocyanate.
 20. The method of claim 11 where the alkyl group contains from 4 to about 10 carbon atoms.
 21. The method of claim 20 where the alkyl group contains from 4 to 8 carbon atoms.
 22. The method of claim 11 where the alkyl is a secondary alkyl moiety.
 23. The method of claim 11 where the alkyl group is the 2-butyl group.
 24. The method of claim 11 where the diamine is a N,N'-dialkyl-4,4'-methylenedianiline.
 25. The method of claim 24 where the diamine is N,N'-di(2-butyl)-4,4'-methylenedianiline.
 26. The method of claim 11 where from about 5% to about 35% of the diamine is replaced by a polyol.
 27. The method of claim 11 where the temperature is from about 75° to about 130° C. 